Dock levelers are used at loading docks to bridge the gap between a loading dock and the bed of a vehicle parked at the loading dock to facilitate the loading or unloading of the vehicle. For example, dock levelers enable material handling equipment, such as a fork truck, to move between the dock and the vehicle bed.
Installation of dock levelers generally falls into two categories: pit-style dock levelers and edge-of-dock levelers. Pit-style dock levelers are installed in a pit or cavity below the level of the dock floor so that the top of the dock leveler is at dock level when stored. Pit-style dock levelers are typically configured to store horizontally, but some pit-style dock levelers store vertically. Examples of pit-style dock levelers are described in U.S. Pat. Nos. 4,068,338 and 4,928,340. Examples of vertical-storing dock levelers are found in U.S. Pat. Nos. 4,825,493 and 5,001,799. Pit-style dock levelers generally have the ability to service transport vehicles up to twelve inches above or below dock level. These dock levelers are typically activated by hydraulic, pneumatic, or mechanical spring systems. Pit-style dock levelers can be expensive to install due to the expense of the pit. Pit-style dock levelers, particularly horizontally-storing pit-style dock levelers, can also have the drawback of being more difficult to ensure security and environmental control.
Edge-of-dock levelers are attached to the dock wall and therefore do not require a pit or shelf for installation. Examples of edge-of-dock levelers can be found in U.S. Pat. Nos. 4,665,579 and 4,689,846. Edge-of-dock levelers are typically smaller and of simpler construction than pit-style dock levelers. A drawback of edge-of-dock levelers is that they can have limited range above and below dock and steeper grades due to their relatively short lengths for fork trucks traveling over them.
A third type of dock installation that is known, but not widely used, is the horizontally-storing top-of-dock leveler. It is typically similar in construction to a horizontally-storing pit-style dock leveler but mounted on top of the dock without a pit. Top-of-dock levelers generally have limited below dock range and are generally used to service standard height trucks from a low dock. Also, top-of-dock levelers are generally used on an indoor dock, otherwise it requires that the door be modified to close properly.
Accordingly, it is desirable to provide a method and apparatus for bridging the gap between a dock and the end of a vehicle bed that can be installed economically such as with an edge-of-dock leveler, but which has greater vertical range and less steep grades than edge-of-dock levelers. It is also to desirable that the method and apparatus provide the security and environmental control of a vertically-storing dock leveler.
U.S. Pat. No. 3,411,168 is an example of a mechanically-operated, vertically-storing dock leveler. The illustrated dock leveler includes a spring counterbalance to assist in operation. The spring counterbalance mechanism includes a spring, a roller, a cam and an arm. The spring counterbalance tends to urge the dock leveler into the stored, vertical position. During operation, as the dock leveler rotates toward an operative position, the weight moment of the dock leveler overcomes the upward bias of the spring counterbalance and the dock leveler falls into its operative position.
U.S. Pat. No. 3,460,175 is another example of a mechanically-operated, vertically-storing dock leveler. The illustrated dock leveler includes both a spring counterbalance and pivotable weight to assist in operation. The spring counterbalance includes a spring, cam, and cam roller. The spring counterbalance upwardly biases the dock leveler, whereas the weight downwardly biases the dock leveler when the weight is rotated into its high moment position. In operation, when the dock leveler is rotated toward the horizontal position, the weight rotates from its low moment position to its high moment position. According to the specification, when the weight is in the high moment position, the moment about the pivotal connection of the deck assembly to the support means is sufficient to overcome the upward biasing action of the spring so that the deck assembly remains in the horizontal position. To re-store the dock leveler, the weight is rotated from the high moment position to the low moment position. According to the specification, when the weight is in the low moment position, the force provided by the spring is sufficient to rotate the dock leveler into the stored, vertical position.
A drawback of the above-described dock levelers is that the spring counterbalance upwardly biases the dock leveler throughout the rotation of the dock leveler. Accordingly, rotation of the dock leveler downward is made more difficult because the motion is against the force of the spring. Another drawback of the above-described dock levelers is that they require additional components such as the arm and cam roller.
Accordingly, it is desirable to provide a spring counterbalance method and apparatus for assisting the operation of a mechanically-operated, vertically-storing dock leveler that can upwardly bias the dock leveler at certain angles of rotation, downwardly bias the dock leveler at other angles of rotation, and/or neutrally bias the dock leveler at yet other angles of rotation. It is also desirable to provide a method and apparatus for assisting the operation of a mechanically-operated, vertically-storing dock leveler of desirable simplicity.
There is a risk that the dock leveler may unexpectedly raise if used with a transport vehicle that has a bed higher than a given working range of a particular dock leveler. As a result some dock levelers may inadvertently raise to an upwardly biased position and raise above the bed of the transport vehicle. This elevated position of the dock leveler would be undesirable for loading freight on and off a vehicle.
Another limitation of some dock levelers is that because the dock leveler is downwardly biased throughout the generally horizontal working range, it therefore requires great effort to lift it from the horizontal position.